The cervical sympathetic nerve is innervated by the cervical sympathetic nerve, which has the function of pupil opening. There are three levels of neurons in the cervical sympathetic nerve, the first of which is located in the sympathetic center of the lower thalamus, which sends out nerve fibers that later pass through the brainstem to the second level of neurons, the ciliary spinal center located in the cervical 8 to thoracic 2 segment of the spinal cord, and then sends out nerve fibers that reach the third level of neurons located in front of the 2nd-3rd cervical vertebrae, which is the the superior cervical sympathetic ganglion. The superior cervical sympathetic ganglion sends out the internal carotid plexus, which forms the cavernous plexus in the cavernous sinus after entering the skull, and its branches reach the ciliary ganglion, which then sends out the ophthalmic branch, which innervates the pupillary opening muscle of the iris and has the function of opening the pupil.